RFeB system magnets were discovered in 1982 by Sagawa (one of the present inventors) and other researchers. The magnets have the characteristic that most of their magnetic characteristics (e.g. residual magnetic flux density) are far better than those of other conventional permanent magnets. Therefore, RFeB system magnets are used in a variety of products, such as driving motors for hybrid or electric automobiles, battery-assisted bicycle motors, industrial motors, voice coil motors (used in hard disk drives or other apparatuses), high-grade speakers, headphones, and permanent magnetic resonance imaging systems.
Earlier versions of the RFeB system magnet had the defect that the coercivity HcJ was comparatively low among various magnetic properties. Later studies have revealed that a presence of a heavy rare-earth element RH within the RFeB system magnet makes reverse magnetic domains less likely to occur and thereby improves the coercivity. The reverse magnetic domain has the characteristic that, when a reverse magnetic field opposite to the direction of magnetization is applied to the RFeB system magnet, it initially occurs in a region near the boundary of a grain and subsequently develops into the inside of the grain as well as to the neighboring grains. Accordingly, it is necessary to prevent the initial occurrence of the reverse magnetic domain. To this end, RH only needs to be present in regions near the boundaries of the grains so that it can prevent the reverse magnetic domain from occurring in the regions near the boundaries of the grains. On the other hand, increasing the RH content unfavorably reduces the residual magnetic flux density Br and consequently decreases the maximum energy product (BH)max. Increasing the RH content is also undesirable in that RH are rare elements and their production sites are unevenly distributed globally. Accordingly, in order to increase the coercivity (and thereby impede the formation of the reverse magnetic domain) while decreasing the RH content to the lowest possible level, it is preferable to make the RH exist at high concentrations in a region near the surface (grain boundary) of the grain rather than in deeper regions.
Patent Literatures 1 and 2 each disclose a method for diffusing RH atoms through the grain boundaries of an RFeB system magnet into regions near the surfaces of the grains by adhering a powder or other forms of material containing an RH or RH compound to the surface of the RFeB system magnet and heating the RFeB system magnet together with the adhered material. Such a method of diffusing RH atoms through the grain boundaries into regions near the grains is called the “grain boundary diffusion method.” An RFeB system magnet before being subjected to the grain boundary diffusion treatment is hereinafter called the “base material” and is distinguished from an RFeB system magnet which has undergone the grain boundary diffusion treatment.
According to Patent Literature 1, a powder or foil containing an RH or RH compound is simply placed on the surface of the base material. Since the adhesion between the powder or foil and the base material is weak, it is impossible to diffuse a sufficient amount of RH atoms into the regions near the surfaces of the grains in the RFeB system magnet. On the other hand, according to Patent Literature 2, a coating material prepared by dispersing a powder of RH or RH compound in an organic solvent is applied to the surface of the base material. Such a coating material can yield a higher adhesion strength to the RFeB system magnet than the powder (singly used) or foil, so that a greater amount of RH atoms can be dispersed into the regions near the surfaces of the grains in the RFeB system magnet.
There are various methods for applying such a coating material to the base material. In a method described in Patent Literature 2, a coating material in the form of slurry prepared by dispersing a powder of RH or RH compound in an organic solvent is applied to the surface of the base material by the technique of screen printing. Specifically, a screen having a permeable area for allowing the coating material to pass through is brought into contact with the surface of the base material. After a coating material is poured onto the surface of the screen from the side opposite to the base material across the screen, a squeegee is slid across that surface of the screen to supply the coating material through the permeable area to the surface of the base material. Consequently, a pattern of the coating material having a shape corresponding to the permeable area is formed on the surface of the base material. It is also possible to simultaneously apply the coating material to a number of base materials by arranging those base materials and providing one screen with a number of permeable areas corresponding to those base materials.
Patent Literature 2 also discloses a method including the steps of applying a coating material to one face of a plate-shaped base material, reversing the base material, and applying the coating material to the opposite face of the base material. In the step of applying the coating material to the opposite face, the base material is placed on a tray consisting of a plate having a hole slightly smaller than the outer shape of the base material, in such a manner that the edge of its material-applied face is supported by the plate surrounding the hole, whereby the applied material is prevented from coming in contact with the tray at the position of the hole. Furthermore, in the heating process for the grain boundary diffusion treatment performed after the application of the coating material, a supporting device with a plurality of pointed projections is used. The base material is placed on these projections, with one of the two material-applied faces directed downward (and accordingly the other face directed upward), whereby the contact between the coating material on the lower face and the supporting device is minimized.
There are three major types of RFeB system magnets: (i) a sintered magnet, which is produced by sintering a raw-material alloy powder mainly composed of the main phase grains; (ii) a bonded magnet, which is produced by molding a raw-material alloy powder with a binder (made of a polymer, elastomer or similar organic material) into a solid shape; and (iii) a hot-deformed magnet, which is produced by performing a hot-deforming process on a raw-material alloy powder. Among these types, the grain boundary diffusion treatment can be performed on (i) the sintered magnet and (iii) the hot-deformed magnet, which do not contain any binder made of an organic material in the grain boundaries.